The invention relates to a method of manufacturing an electrically conductive adhesive bond between electrically conductive parts whose surfaces to be bonded exhibit a certain degree of surface roughness.
In microelectronics, electric connections between, for example, components and printed circuit boards or sub-carriers in the form of glass or ceramic plates are formed by microwelding, soft soldering and glueing. In contrast with the necessarily sequential microwelding process, soldering and glueing processes have the advantage that all connections at one circuit level can be formed substantially at the same time.
In microelectronics adhesive bonds are formed with electrically conductive adhesives which are filled with up to 70% by volume of metal powder, for example silver powder, to obtain reliable electric connections. The adhesive establishes the mechanical connection between the contact surfaces to be bonded; the metal particles suspended in the adhesive contact one another and thus provide electric conduction.
A disadvantage of the use of adhesives which are filled with metal powder is that they must be applied very accurately when the parts to be bonded have small dimensions, and moreover, short-circuits may develop between closely spaced terminal areas. For this reason, the microelectronics industry predominantly employs soft-soldering methods which have the additional economical advantage that the coating process, which is necessary to obtain surfaces which can be subjected to soft-soldering and which consists of the electro-deposition of tin-coated copper layers, can be carried out readily and reproducibly accurate, even when the dimensions are small, and because the price of commercially available soft solder is low. Thoroughly prepurified surfaces are a prerequisite for the use of metal-powder-filled bonding agents which, in addition, are relatively expensive (at present DM 1000/kg). Consequently, their use is limited to very high-quality and/or tmperature-sensitive parts of circuits.